Adding PHP examples

04_quicksort/php/01_loop_sum.php

@@ -0,0 +1,12 @@
+<?php
+
+function sum(array $arr)
+{
+    $total = 0;
+    for ($x = 0; $x < count($arr); $x++) {
+        $total += $arr[$x];
+    }
+  return $total;
+}
+
+echo  sum([1, 2, 3, 4]); // 10

04_quicksort/php/02_recursive_count.php

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+<?php
+
+function sum($list)
+{
+    if (count($list) === 0) {
+        return 0;
+    }
+  return $list[0] + sum(array_splice($list, 1));
+}
+
+echo sum([1, 2, 3, 4]); // 10

04_quicksort/php/03_recursive_count.php

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+<?php
+
+function recursiveCount($list) {
+    if (count($list) === 0) {
+        return 0;
+    }
+  return 1 + recursiveCount(array_splice($list, 1));
+}
+
+echo count([0, 1, 2, 3, 4, 5]); // 6

04_quicksort/php/04_recursive_max.php

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+<?php
+
+function recursiveMax($list) {
+    if (count($list) === 2) {
+        return $list[0] > $list[1] ? $list[0] : $list[1];
+    }
+  $subMax = recursiveMax(array_splice($list, 1));
+  return $list[0] > $subMax ? $list[0] : $subMax;
+}
+
+echo recursiveMax([1, 5, 10, 25, 16, 1]); // 25

04_quicksort/php/05_quicksort.php

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+<?php
+
+function quicksort(array $array) {
+    if (count($array) < 2) {
+        // base case, arrays with 0 or 1 element are already "sorted"
+        return $array;
+    } else {
+        // recursive case
+        $pivot = $array[0];
+        var_dump($array);
+
+        // sub-array of all the elements less than the pivot
+        $less = array_filter(array_slice($array, 1), function($el) use ($pivot) { return $el <= $pivot; });
+
+        // sub-array of all the elements greater than the pivot
+        $greater = array_filter(array_slice($array, 1), function($el) use ($pivot) { return $el > $pivot; });
+        return array_merge(quicksort($less), [$pivot], quicksort($greater));
+  }
+}
+
+var_dump(quicksort([10, 5, 2, 3])); // [2, 3, 5, 10]